Derivation of kinetic rate constant of enzyme-buffer mud cake clean up systems - Laboratory investigation and verification in high temperature fractured carbonate reservoir

Horizontal wells enable drainage from a longer wellbore which helps to allow lower drawdown rate compared to vertical wells, minimizing gas or water coning. However productivity can be seriously affected unless mud cake damage is efficiently removed from all producing intervals along the horizontal wellbore. In recent years eco-friendly and non-corrosive bio-enzymes (α/β- amylase) have shown great potential in cleaning wellbores uniformly and achieving higher well productivity. However in a low pressure fractured reservoir, there is always a possibility of localized reaction and loss of the clean-up fluid, unless the reactivity of the fluid is engineered based on the given well parameters. In this study α-amylase enzyme is modified to withstand higher thermal shock by structurally reinforcing the β-Helix layer to strengthen the catalytic centre by preferential protein hydration technique. Buffering was done to maintain different system pH and kinetic rate constant is derived through reducing sugar release measurement by DNS method using starch-xanthan gum-CaCO₃ based drill-in-fluid as substrate. Though the overall reaction is extremely complex, a good correlation could be drawn between system pH and the rate of breaking mud cake into simple sugar. The kinetic rate constant index is used in final formulation of enzymatic clean up fluid for application in high temperature (110°C) long horizontal wells drilled in carbonate formation, which allowed different soaking time due to operational constraints. The results show that there is excellent correlation between laboratory prediction and clean up efficiency in terms of well productivity. The study showed that each individual well demands a specific formula for clean-up fluid and higher than prognosed production could be achieved through custom formulation, based on well condition and operational requirement.